Plasma position control for extreme ultraviolet lithography light sources

ABSTRACT

A method includes dispensing a droplet into a vacuum chamber; firing a pre-pulse laser to the droplet; sensing a first image of a return beam of the pre-pulse laser from the droplet; after firing the pre-pulse laser, firing a main-pulse laser to the droplet, wherein when the main-pulse laser hits the droplet, the droplet is vaporized into a plasma that emits extreme ultraviolet radiation; after sensing the first image and firing the main-pulse laser, sensing a second image of a return beam of the main-pulse laser from the droplet; and adjusting a plasma position in the vacuum chamber according to at least the second image.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation application of U.S. Pat. ApplicationNo. 16/900,735, filed Jun. 12, 2020, which is herein incorporated byreference in its entirety.

BACKGROUND

Extreme ultraviolet (EUV) lithography is an optical lithographytechnique in which the scanner uses light in the extreme ultravioletregion (e.g., spanning wavelengths of approximately one to one hundrednanometers). One specific type of EUV lithography, referred to aslaser-produced plasma (LPP) EUV lithography, uses a light source toconvert a molten metal such as tin into a highly ionized plasma thatemits EUV radiation. An LPP EUV light source may generally include avacuum chamber, a laser power source (e.g., a carbon dioxide laser), anda droplet generator. The droplet generator dispenses droplets of themolten metal into the vacuum chamber, and, when the droplets reach apredefined position in the vacuum chamber, the laser power source firesa series of laser pulses into the vacuum chamber. When the laser pulseshit a droplet, the droplet is vaporized into a plasma which emits theEUV radiation.

The EUV radiation is subsequently guided, using a series of optics(e.g., including multilayer mirrors), into the scanner, and then used toproject a pattern (etched into a photomask) onto a silicon wafer. TheEUV process can be used to fabricate a high resolution pattern of linesonto the silicon wafer, potentially at a scale of seven nanometers orbeyond.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram illustrating one example of an extremeultraviolet lithography light source, according to examples of thepresent disclosure;

FIG. 2 is a schematic diagram illustrating one example of the firstfeedback generator of FIG. 1 ;

FIG. 3 is a flow diagram illustrating a method for controlling theposition of a plasma in an extreme ultraviolet lithography light source,according to one example of the present disclosure; and

FIG. 4 is a flow diagram illustrating a method for controlling theposition of a plasma in an extreme ultraviolet lithography light source,according to another example of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature’s relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In one example, the present disclosure provides plasma position controlfor extreme ultraviolet (EUV) light sources. As discussed above, an EUVlight source may generally include a vacuum chamber, a laser powersource, and a droplet generator. The droplet generator dispensesdroplets of molten metal (e.g., tin) into the vacuum chamber, and, whenthe droplets reach a predefined position in the vacuum chamber, thelaser power source fires a series of laser pulses into the vacuumchamber. When a first pulse (also referred to as a “pre-pulse”) hits adroplet, the generally spherical droplet takes on a flattened,pancake-like shape. When a second pulse (also referred to as a “mainpulse”) hits the pancake, the pancake is vaporized into a plasma whichemits EUV radiation. The EUV radiation may subsequently be used toproject a pattern formed in a photomask onto a silicon wafer, and thepattern may be etched into the wafer.

The precision with which the laser hits the droplets is a majordetermining factor in the conversion efficiency of the EUV light source(i.e., how much of the laser power is converted into EUV radiation).State of the art LPP light sources have been shown to exhibit conversionefficiencies of less than ten percent. Thus, these light sources consumea large amount of energy, but manage to produce a relatively smallamount of light for the power consumed. Conventional LPP light sourcesinclude control systems to fix the droplet position and the position ofthe laser relative to the droplet individually, but do not allow for theplasma position (e.g., the position in the vacuum chamber at which themain pulse hits the pancake to produce the plasma) to be fixed. Thus,over time, conditions such as drift of the laser beam or changes in theplasma position may affect the precision with which the laser hits thedroplets, and therefore reduce the conversion efficiency of the lightsource.

Examples of the present disclosure provide feedback controls to fix theplasma position in the vacuum chamber of an EUV light source. In oneexample, in addition to calculating the droplet position from the returnbeam of the pre-pulse (i.e., the first laser pulse that hits thespherical droplet) as is conventionally done, the present disclosurealso calculates the droplet position from the return beam of the mainpulse (i.e., the second laser pulse that hits the pancake). The x, y,and z coordinates extracted from the return beams may be used asfeedback to control the laser power source. For instance, the xcoordinate (i.e., the position along a line that is parallel to thedirection in which the droplets flow) may be used to adjust a time delaybetween the pre-pulse and the laser pulse, which consequently willadjust the position at which the pancake is vaporized into plasma (e.g.,by allowing the pancake to fall a shorter distance or a longer distancebefore vaporization). In addition, the y and z coordinates may be usedto adjust the optics (e.g., mirrors) that are used to steer the beams ofthe laser pulses relative to the droplet and the pancake. Theadjustments of the time delay and the beam steering optics, based onobserved plasma positions within the vacuum chamber, may allow theplasma position to remain fixed at some target position. In other words,the adjustments may work to continuously minimize a delta or differencebetween the observed plasma position and the target plasma position, sothat the plasma position remains fixed and does not drift.

Within the context of the present invention, the term “droplet” isunderstood to refer to the molten metal (e.g., tin) that is dispensedinto the vacuum chamber of an EUV light source before the molten metalis hit with a laser pulse. The term “pancake” is understood to refer tothe molten metal after the molten metal is hit with a first laser pulse,but before the molten metal is converted to plasma (e.g., after themolten metal is hit with a pre-pulse, but before the molten metal is hitwith a main pulse). Moreover, the term “observed plasma position” isunderstood to refer to a position at which the laser is observed (e.g.,by a camera or other sensor) to hit the pancake. The term “target plasmaposition” is a predefined position at which it is desired for the laserto hit the pancake. Examples of the present disclosure may attempt tomaintain a fixed plasma position by continuously monitoring the observedplasma position and making adjustments to the EUV light source asnecessary to ensure that the plasma position is as close to the targetplasma position as possible.

FIG. 1 is a schematic diagram illustrating one example of an extremeultraviolet lithography light source 100, according to examples of thepresent disclosure. The discussion of FIG. 1 makes references topositions in an x,y,z coordinate plane. In one example, the x axis ofthe coordinate plane is defined along a line that is parallel to thedirection in which the droplets travel (or perpendicular to thedirection in which the laser pulses propagate). The z axis of thecoordinate plane is defined along a line that is parallel to thedirection in which the laser pulses propagate (or perpendicular to thedirection in which the droplets travel). The y axis of the coordinatesystem is defined along a line that is perpendicular to both the x axisand the z axis.

As discussed above, the system 100 generally comprises a vacuum chamber102, a laser power source 104, and a droplet generator 106.

The droplet generator 106 comprises a vessel that is configured todispense a continuous stream of microscopic droplets, where the dropletsare formed of a metal material in either liquid or solid form. Forinstance, in one example, the droplets are formed from molten tin.However, in other examples, the droplets may be formed from othermaterials, such as tin-containing liquid materials (e.g., eutecticalloys containing tin, lithium, and xenon).

In one example, the droplet generator 106 may include a gas supplier orother system (not shown) that is configured to pump a gas that forcesthe droplets out of the droplet generator 106 (e.g., via a nozzle). Inthis case, the flow velocity of the continuous stream of droplets may bea function of the pressure of the gas being pumped. For instance, whenthe gas pressure is increased, the flow velocity may be faster;conversely, when the gas pressure is decreased, the flow velocity may beslower.

In one example, each droplet may have a diameter of approximatelytwenty-five to thirty microns, and the droplet generator 106 maydispense the droplets at a rate of approximately 50,000 droplets persecond, or fifty kilohertz.

The droplet generator 106 is coupled to the vacuum chamber 102. Thevacuum chamber 102 comprises an interior volume that is maintained in avacuum environment. The droplet generator 106 may dispense thecontinuous stream of droplets into the vacuum environment of the vacuumchamber 102. For instance, the droplet generator 106 may be positionednear an inlet on the top of the vacuum chamber 102 and may dispense thedroplets into the vacuum chamber 102 so that the droplets fall towardthe bottom of the vacuum chamber 102. As discussed above, the dropletsdispensed by the droplet generator 106 may have a generally sphericalform, as shown by the droplet 120.

The laser power source 104 is also coupled to the vacuum chamber 102. Inone example, the laser power source 104 generally comprises a seed laser108, a set of power amplifiers 110, and a beam transport system (BTS)112. In one example, the seed laser 108 is a twenty to thirty kilowattcarbon dioxide (CO₂) laser source. However, in other examples, the seedlaser 108 may comprise another type of laser source, such as aneodymium-doped yttrium aluminum garnet (Nd:YAG) laser source. The seedlaser 108 may comprise a single laser or multiple lasers. The seed laser108 is configured to generate a plurality of laser pulses, including apre-pulse and a main pulse. The laser pulses may have a specific spotsize (e.g., approximately 100 to 300 microns). In one example, the laserpulses are generated by the seed laser 108 at the same rate that thedroplets are dispensed by the droplet generator 106. However, asdiscussed in greater detail below, the rate at which the laser pulsesare generated may be adjusted to maintain a fixed plasma position withinthe vacuum chamber 102. In another example, the seed laser 108 mayinclude a pre-amplifier (e.g., a high gain carbon dioxide amplifier)which may perform an initial amplification of the laser pulses (e.g., upto a magnitude of approximately 100 watts).

In one example, the set of power amplifiers 110 comprises a plurality ofamplifiers that amplify, in a plurality of stages, the laser pulsesgenerated by the seed laser 108. In one example, the laser pulses may beamplified to up to tens of kilowatts of average pulse power and multiplemegawatts of peak pulse power. Thus, the set of power amplifiers 110 mayprovide most of the amplification that is needed to reach power levelsthat are high enough to generate EUV radiation in the vacuum chamber102.

In one example, the BTS 112 comprises a set of optics (e.g., a periscopeand/or mirrors, including M120 and/or M150 mirrors, which may be coatedto minimize absorption of light from the laser) that is configured toexpand the beams of the laser pulses and to steer the beams to aspecific, predefined position in the vacuum chamber 102 (e.g., towardthe stream of droplets). In one example, the BTS 112 may transport thebeams up to thirty meters in the direction of the vacuum chamber 102.Thus, the beams of the laser pulses are steered or controlled tomaximize the generation of EUV radiation. In one example, the BTS 112may also include a second feedback generator 124 that adjusts the BTS112 (e.g., adjusts the angle and/or direction in which the beams aresteered) in an effort to maintain a fixed plasma position in the vacuumchamber, as discussed in greater detail below.

In one example, the laser power source 104 may further comprise acontroller 126. The controller 126 may adjust the power of the laserlight source 104 (e.g., by adjusting the output of the set of poweramplifiers 110) in response to an intensity of photons emitted by theplasma in the vacuum chamber 102. For instance, the controller 126 mayadjust the amplifiers to increase or decrease the amplification of thelaser pulses emitted by the seed laser 108.

As discussed above, the vacuum chamber 102 comprises a vessel or volumewithin which the EUV radiation is generated. The vacuum chamber 102 mayinclude one or more sensors 114. In one example, the sensors 114 mayinclude a plurality of cameras positioned in different locations withinthe vacuum chamber 102. In this case, the plurality of cameras mayinclude multiple different types of cameras that are configured andpositioned to capture different types of information. For instance, thevacuum chamber may include a coarse droplet source camera (CDSC) tocapture images of droplet locations within the vacuum chamber 102, afine droplet source camera (FDSC) to capture images of pancakes withinthe vacuum chamber 102, and one or more other types of cameras. In oneexample, the sensors 114 are positioned to monitor the positions of thepre-pulse and the main pulse in the vacuum chamber, i.e., where thepre-pulse hits the droplet 120 and where the main pulse hits the pancake122. For instance, where the sensors 114 include cameras, the camerasmay capture images of the pre-pulse and the main pulse as they hit thedroplet 120 and the pancake 122, respectively.

The system 100 comprises further elements that assist in controlling theplasma position (i.e., the position at which the pancake 122 isvaporized) within the vacuum chamber 102. In one example, these furtherelements include a first feedback generator (FG) 116 and a controller118.

FIG. 2 is a schematic diagram illustrating one example of the firstfeedback generator 116 of FIG. 1 . In one example, the first feedbackgenerator 116 comprises a filter 200, an amplifier 202, and aposition-sensitive detector 204. The filter 200 may be configured toextract a pancake return beam from images provided by the sensor 114. Inone example, the pancake return beam comprises laser light that isreflected by the pancake 122 when the main pulse hits the pancake 122.In one example, the filter 200 may extract the pancake return beam froman image by filtering the light from the laser pre-pulse and the lightfrom the laser main pulse from the image. In one example, the filter 200may comprise a mirror having a coating, such as a dichroic mirror. Oncethe pre-pulse light and main pulse light have been filtered from theimage, the image may be delivered to the amplifier 202, which amplifiesthe return beam in the image. The position-sensitive detector 204receives the amplified and filtered image and determines, based on theoptical beam deflection in the image, the x,y,z coordinates of theobserved plasma position in the vacuum chamber 102. In one example, theposition-sensitive detector 204 is a quad-cell photodetector array.

Referring back to FIG. 1 , the position-sensitive detector 204 mayprovide the x coordinate of the observed plasma position to thecontroller 118. In turn, the controller 118 may adjust the time delaybetween the pre-pulse and the main pulse, based on the location of theobserved plasma position’s x coordinate relative to the target plasmaposition’s x coordinate. For instance, if the x coordinate of theobserved plasma position is the same (or is within some acceptablepredefined tolerance of) the x coordinate of the target plasma position,then the controller 118 may leave the time delay as it is. However, ifthe x coordinate of the observed plasma position is not the same (or isnot within the acceptable predefined tolerance of) the x coordinate ofthe target plasma position, then the controller 118 may shorten orlengthen the time delay as necessary until the x coordinate of theobserved plasma position is the same (or is within the acceptablepredefined tolerance of) the x coordinate of the target plasma position.

Adjustment of the time delay may therefore adjust the position (i.e., atleast the x position) of the pancake relative to the plasma. As aresult, the position at which the main pulse hits the pancake (i.e., theplasma position) may be adjusted simply by controlling the time at whichthe main pulse is fired relative to the pre-pulse (which may allow thepancake to fall a slightly shorter distance or a slightly longerdistance before the pancake is vaporized). Thus, the position-sensitivedetector 204 and the controller 118 may comprise a first feedback loopthat allows the plasma position to be adjusted without adjusting aposition of the laser power source 104 or the droplet generator 106. Inone example, the controller 118 may comprise a quad-cell timing loop.

The position-sensitive detector 204 may also provide the y and zcoordinates of the observed plasma position to the BTS 112 in a secondfeedback loop. The second feedback generator 124 of the BTS 112 mayadjust the BTS 112 in response to the y and z coordinates of theobserved plasma position (e.g., by adjusting the angles and/or positionsof one or more mirrors used to guide the beams, such as the M120 and/orM150 mirrors of the BTS 112). For instance, if the y and z coordinatesof the observed plasma position are the same as (or are within anacceptable predefined tolerance of) the y and z coordinates of thetarget plasma position, then the BTS 112 may not be adjusted. However,if the y and z coordinates of the observed plasma position are not thesame as (or are not within an acceptable predefined tolerance of) the yand z coordinates of the target plasma position, then the secondfeedback generator 124 of the BTS 112 may adjust the BTS 112 in the yand/or z axis so that the beams delivered by the BTS 112 into the vacuumchamber 102 hit the pancakes at the target plasma position. Thus, theplasma position can be further fixed by adjusting the positions of thelaser beams in the y and z axes relative to the droplets and pancakes.

However, adjusting the y and/or z positions of the beams may change thelocations in which portions of the droplets which are not vaporizedfall. That is, as the stream of droplets flows into the vacuum chamber102 and is vaporized by the laser pulses, some portions of the dropletsmay fail to be vaporized (e.g., a laser pulse may miss all or a portionof a droplet despite the best efforts to precisely control the plasmaposition). The portions of a droplet that are not vaporized may fall tothe bottom of the vacuum chamber 102, where the portions of the dropletmay be caught by a droplet catcher (not shown). The droplet catchercollects the portions of the droplets that fail to be vaporized, so thatthese portions of the droplets do not build up on the interior surfacesof the vacuum chamber 102. Buildup of molten metal on the interiorsurfaces of the vacuum chamber 102 may degrade the performance of thesystem 100 and reduce the conversion efficiency.

In one example, the vacuum chamber 102 may further include a dropletgenerator (DG) metrology 128. The droplet generator metrology 128 mayobserve the metrology of the droplets that are dispensed by the dropletgenerator 106 (e.g., droplet 120 and the like). More specifically, thedroplet generator metrology 128 may observe the y and z positions of thedroplets and may control the position of the droplet generator 106 toensure that the droplets are not dispensed in a position where anyportions of the droplets that are not vaporized may land outside of thedroplet catcher. Thus, the droplet generator metrology 128 may providefeedback to the droplet generator 106 in a third feedback loop to ensurethat any adjustments made to the BTS 112 do not result in excess dropletbuildup outside of the droplet catcher.

It will be appreciated that FIG. 1 represents a simplified form of a EUVlight source 100. In some examples, the EUV light source 100 may includeadditional components that are not illustrated, such as a collector anda droplet catcher. The collector may comprise a multilayer mirror off ofwhich the photons of the EUV radiation are reflected and directedthrough an intermediate focus unit, which is positioned outside of thevacuum chamber 102. The droplet catcher may be positioned to collectportions of the droplets that are not vaporized by the laser pulses andthat fall to the bottom of the vacuum chamber 102.

FIG. 3 is a flow diagram illustrating a method 300 for controlling theposition of a plasma in an extreme ultraviolet lithography light source,according to one example of the present disclosure. The method 300 maybe performed using one or more different components of a laser producedplasma EUV light source, under the control of controller or processor.

The method 300 begins in step 302. In step 304, a continuous flow ofmetal droplets may be dispensed into a vacuum chamber. As discussedabove, the droplets may be spherical in shape and may have a diameter ofapproximately twenty-five to thirty microns. The droplets may bedispensed at a rate of approximately 50,000 droplets per second, orfifty kilohertz. The droplets may comprise tin or a tin-containingliquid material (e.g., eutectic alloys containing tin, lithium, andxenon) in liquid or solid form.

In step 306, a plurality of laser pulses (including at least a first anda second pulse) may be fired into the vacuum chamber. As discussedabove, the plurality of laser pulses may be generated from a twenty tothirty kilowatt carbon dioxide CO₂ laser source and may be amplifiedprior to entering the vacuum chamber. The plurality of laser pulses mayhave a specific spot size (e.g., approximately 100 to 300 microns). Whenthe first laser pulse hits a droplet in the vacuum chamber, thepreviously spherical droplet takes on a pancake-like shape. When thesecond laser pulse hits the pancake, the pancake is vaporized into ahighly ionized plasma that emits EUV radiation.

In step 308, the laser light source may be adjusted to maintain a plasmaposition within the vacuum chamber at a fixed position during subsequentdispensing and firing operations. As discussed above, the plasmaposition is a position within the vacuum chamber at which the dropletsare vaporized to form the plasma.

The method 300 may then return to step 304 and continue as describedabove. Thus, the method 300 may continuously loop through the steps304-308 during an EUV lithography process, to ensure that the plasmaposition remains fixed throughout the EUV lithography process.

FIG. 4 is a flow diagram illustrating a method 400 for controlling theposition of a plasma in an extreme ultraviolet lithography light source,according to another example of the present disclosure. The method 400may be viewed as a more detailed version of the method 300, discussedabove. Thus, the method 400 may be performed using one or more differentcomponents of a laser produced plasma EUV light source, under thecontrol of controller or processor.

The method 400 begins in step 402. In step 404, a continuous flow ofmetal droplets may be dispensed into a vacuum chamber. As discussedabove, the droplets may be spherical in shape and may have a diameter ofapproximately twenty-five to thirty microns. The droplets may bedispensed at a rate of approximately 50,000 droplets per second, orfifty kilohertz. The droplets may comprise tin or a tin-containingliquid material (e.g., eutectic alloys containing tin, lithium, andxenon) in liquid or solid form.

In step 406, a first laser pulse (e.g., a pre-pulse) may be fired intothe vacuum chamber. As discussed above, the first laser pulse may begenerated from a twenty to thirty kilowatt carbon dioxide CO₂ lasersource and may be amplified prior to entering the vacuum chamber. Thefirst laser pulse may have a specific spot size (e.g., approximately 100to 300 microns) When the first laser pulse hits a droplet in the vacuumchamber, the previously spherical droplet takes on a pancake-like shape.

In step 408, a first image of a return beam from the droplet (e.g., aportion of the laser light from the first laser pulse that is reflectedupon impact with the droplet) may be captured. In one example, the firstimage may be captured by a camera that is positioned within the vacuumchamber, such as a coarse droplet source camera.

In step 410, a second laser pulse (e.g., a main pulse) may be fired intothe vacuum chamber, after a delay (where the delay defines a timedifference between the first laser pulse and the second laser pulse).Like the first laser pulse, the second laser pulse may be generated froma twenty to thirty kilowatt carbon dioxide CO₂ laser source and may beamplified prior to entering the vacuum chamber. The second laser pulsemay have a specific spot size (e.g., approximately 100 to 300 microns).When the second laser pulse hits the pancake, the pancake is vaporizedinto a highly ionized plasma that emits EUV radiation.

In step 412, a second image of a return beam from the pancake (e.g., aportion of the laser light from the first and/or second laser pulse thatis reflected upon impact with the pancake) may be captured. Thus, thesecond image may depict an observed plasma position in the vacuumchamber. In one example, the second image may be captured by a camerathat is positioned within the vacuum chamber, such as a fine dropletsource camera.

In step 414, x, y, and z coordinates of the observed plasma position maybe determined, based on an analysis of at least the second image. Forinstance, in one example, the second image may be processed to removelight from the first and second laser pulses and to amplify the returnbeam from the pancake. The x, y, and z coordinates of the observedplasma position may then be determined based on optical beam deflection.The x coordinate may represent a position of the observed plasmaposition along a line that is parallel to the direction in which thedroplets flow. The z coordinate may represent a position of the observedplasma position along a line that is parallel to the propagation of thelaser pulses. The y coordinate may represent a position of the observedplasma position along a line that is perpendicular to both the x axisand the z axis.

In step 416, a first adjustment to the delay between the first laserpulse and the second laser pulse may be calculated, where the firstadjustment minimizes a difference in the x position between the observedplasma position and a target plasma position. In one example, where thedifference in x position is zero or is within some predefined tolerance,the first adjustment may not be necessary.

In step 418, a second adjustment to a beam transport system that steersthe beams of the first and second laser pulses may be calculated, wherethe second adjustment minimizes a difference in the y,z position betweenthe observed plasma position and the target plasma position. In oneexample, the second adjustment may involve making adjustments to thepositions and/or angles of one or more mirrors of the beam transportsystem (e.g., an M120 and/or M150 mirror). In one example, where thedifference in y,z position is zero or is within some predefinedtolerance, the second adjustment may not be necessary.

In step 420, at least one of the first adjustment and the secondadjustment is applied to the EUV light source. For instance, applicationof the first adjustment may result in a change to the time delay betweenthe firing of the first laser pulse and the second laser pulse (e.g.,the time delay may be made shorter or longer). Application of the secondadjustment may result in the angle and/or direction at which the beamsof the first laser pulse and the second laser pulse are steered into thevacuum chamber. As discussed above, one or both of the first adjustmentand the second adjustment may not be necessary at any given time.

The method 400 may then return to step 404 and continue as describedabove. Thus, the method 400 may continuously loop through the steps404-420 during an EUV lithography process, to ensure that the plasmaposition remains fixed at the target plasma position throughout the EUVlithography process. As noted above, some iterations of the method 400may result in no adjustments to the EUV light source. For instance, thefirst adjustment may be determined to be necessary, while the secondadjustment may be determined to not be necessary, or vice versa.Alternatively, neither of the first adjustment and the second adjustmentmay be determined to be necessary if the plasma position has not changed(or has changed within some predefined tolerance). However bycontinuously monitoring the observed plasma position, it can be quicklydetermined when any adjustments are necessary, and the adjustments maybe made in a timely manner to maintain the fixed plasma position.

It should be noted that the methods 300 and 400 may be expanded toinclude additional steps or may be modified to include additionaloperations with respect to the steps outlined above. In addition,although not specifically specified, one or more steps, functions, oroperations of the methods 300 and 400 may include a storing, displaying,and/or outputting step as required for a particular application. Inother words, any data, records, fields, and/or intermediate resultsdiscussed in the methods 300 and 400 can be stored, displayed, and/oroutputted either on the device executing the method or to anotherdevice, as required for a particular application. Furthermore, steps,blocks, functions or operations of the above described methods 300 and400 can be combined, separated, and/or performed in a different orderfrom that described above, without departing from the examples of thepresent disclosure.

Thus, examples of the present disclosure provide plasma position controlfor EUV light sources. The control techniques described above may beimplemented to main the plasma position (e.g., the position at which themain pulse hits the pancake to produce the plasma) at a fixed positionwithin a vacuum chamber. By maintaining this fixed position, the effectsof conditions which may reduce the conversion efficiency of the lightsource (such as drift of the laser beams or changes in the plasmaposition) can be minimized.

In one example, the present disclosure provides a system that includes avacuum chamber, a droplet generator, a laser light source, a sensor, anda first feedback loop. The droplet generator may dispense a stream ofdroplets into the vacuum chamber, where the droplets may be formed froma metal material. The laser light source may fire a plurality of laserpulses, including at least a first pulse and a second pulse, into thevacuum chamber. The sensor may detect an observed plasma position withinthe chamber, where the observed plasma position is a position at whichthe plurality of laser pulses vaporizes a droplet of the stream ofdroplets to produce a plasma. The plasma emits extreme ultravioletradiation which may be used to etch a semiconductor wafer. The firstfeedback loop may connect the sensor to the laser light source and mayadjust a time delay between the first pulse and the second pulse. Theadjustment to the time delay may minimize a difference between theobserved plasma position and a target plasma position.

In another example, the present disclosure provides a system thatincludes a vacuum chamber, a droplet generator, a laser light source, asensor, and a feedback loop. The droplet generator may dispense a streamof droplets into the vacuum chamber, where the droplets may be formedfrom a metal material. The laser light source may fire a plurality oflaser pulses, including at least a first pulse and a second pulse, intothe vacuum chamber. The sensor may detect an observed plasma positionwithin the chamber, where the observed plasma position is a position atwhich the plurality of laser pulses vaporizes a droplet of the stream ofdroplets to produce a plasma. The plasma emits extreme ultravioletradiation which may be used to etch a semiconductor wafer. The firstloop may connect the sensor to the laser light source and may adjust atrajectory of the plurality of laser pulses. The adjustment to thetrajectory may minimize a difference between the observed plasmaposition and a target plasma position.

In another example, a stream of droplets is dispensed into a vacuumchamber. The droplets are formed from a metal material. A plurality oflaser pulses is fired at the stream of droplets, using a laser lightsource. When the plurality of laser pulses hits the droplets, thedroplets are vaporized into a plasma that emits extreme ultravioletradiation. The laser light source is adjusted to maintain a plasmaposition within the vacuum chamber that is fixed. The plasma position isa position within the vacuum chamber at which the droplets arevaporized.

The advanced lithography process, method, and materials described in thecurrent disclosure can be used in many applications, including fin-typefield effect transistors (FinFETs). For example, the fins may bepatterned to produce a relatively close spacing between features, forwhich the above disclosure is well suited. In addition, spacers used informing fins of FinFETs can be processed according to the abovedisclosure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: dispensing a droplet into avacuum chamber; firing a pre-pulse laser to the droplet; sensing a firstimage of a return beam of the pre-pulse laser from the droplet; afterfiring the pre-pulse laser, firing a main-pulse laser to the droplet,wherein when the main-pulse laser hits the droplet, the droplet isvaporized into a plasma that emits extreme ultraviolet radiation; aftersensing the first image and firing the main-pulse laser, sensing asecond image of a return beam of the main-pulse laser from the droplet;and adjusting a plasma position in the vacuum chamber according to atleast the second image.
 2. The method of claim 1, wherein when thepre-pulse laser hits the droplet, a shape of the droplet transforms fromspherical to a shape of a pancake.
 3. The method of claim 1, whereinadjusting the plasma position in the vacuum chamber comprises adjustinga time delay between the pre-pulse laser and the main-pulse laser. 4.The method of claim 1, wherein adjusting the plasma position in thevacuum chamber comprises adjusting optics that are used to steer themain-pulse laser relative to the droplet.
 5. The method of claim 1,further comprising: amplifying a power of the pre-pulse laser.
 6. Themethod of claim 1, further comprising: amplifying a power of themain-pulse laser.
 7. The method of claim 1, wherein a spot size of thepre-pulse laser is approximately 100 to 300 microns.
 8. A methodcomprising: providing a droplet from a droplet generator toward adroplet catcher of an extreme ultraviolet lithography light source;firing a main-pulse laser to hit the droplet and form a plasma thatemits extreme ultraviolet radiation in a vacuum chamber of the extremeultraviolet lithography light source; detecting a first image of thedroplet when the main-pulse laser hits the droplet; filtering themain-pulse laser from the first image to extract a return beam of themain-pulse laser; and determining x,y,z coordinates of the plasma basedon the return beam of the main-pulse laser.
 9. The method of claim 8,wherein filtering the main-pulse laser from the first image is furtherto filter light from the main-pulse laser from the first image.
 10. Themethod of claim 8, wherein determining the x,y,z coordinates of theplasma is performed after filtering the main-pulse laser from the firstimage.
 11. The method of claim 8, further comprising after determining ay component of the x,y,z coordinates of the plasma, adjusting opticsthat are used to steer the main-pulse laser relative to the droplet,wherein the y component is defined along a line that is perpendicular toa direction of travel of the droplet.
 12. The method of claim 8, furthercomprising after determining an x component of the x,y,z coordinates ofthe plasma, adjusting a time the main-pulse laser hitting the droplet,wherein the x component is defined along a line that is parallel to adirection of travel of the droplet.
 13. The method of claim 8, furthercomprising: prior to firing the main-pulse laser, firing a pre-pulselaser to hit the droplet.
 14. The method of claim 13, furthercomprising: prior to detecting the first image of the droplet, detectinga second image of the droplet when the pre-pulse laser hits the droplet.15. A method comprising: dispensing a plurality of droplets from adroplet generator toward a droplet catcher of an extreme ultravioletlithography light source; firing a main-pulse laser to hit one of thedroplets to form a plasma that emits extreme ultraviolet radiation in avacuum chamber of the extreme ultraviolet lithography light source;detecting a position of the plasma when the main-pulse laser hits saidone of the droplets; adjusting a position of the droplet generatoraccording to the detected position of the plasma; after adjusting theposition of the droplet generator, observing a position of the dropletsin the vacuum chamber; and adjusting a position of the droplet catcheraccording to the observed position of the droplets in the vacuumchamber.
 16. The method of claim 15, further comprising: prior to firingthe main-pulse laser, firing a pre-pulse laser to hit said one of thedroplets.
 17. The method of claim 16, further comprising: prior todetecting the position of the plasma, detecting a position of said oneof the droplets when the pre-pulse laser hits the said one of thedroplets.
 18. The method of claim 15, further comprising: adjusting atime firing the main-pulse laser according to the detected position ofthe plasma.
 19. The method of claim 15, further comprising: amplifying apower of the main-pulse laser.
 20. The method of claim 15, whereinadjusting the position of the plasma comprises adjusting optics that areused to steer the main-pulse laser relative to the droplets.